The present invention relates to communication systems and methods and more particularly to a system and method for optimizing the bandwidth of a point to multipoint wireless system by synchronizing transmit and receive modes.
Wireless radio links have increasingly become important to provide data communication links for a variety of applications. For example, Internet Service Providers have begun to utilize wireless radio links within urban settings to avoid the installation expense of traditional wired connections or optical fiber. It may be advantageous to utilize wireless radio link systems to provide service to a plurality of users in a point to multipoint architecture. Point to multipoint systems typically consist of a plurality of hub units servicing a plurality of sub units (sometimes referred to as remote units, nodes, or subscriber units). The subs are typically associated with individual nodes on the system. For example, an individual sub unit may be connected to LAN to allow PC's on the LAN to bridge to other networks via the point to multipoint system. Each sub unit communicates via a wireless channel with a particular hub unit. In a point to multipoint system, the hub unit may control communication between a portion of the plurality of sub units associated with a particular coverage area. The hub units schedule transmit and receive bursts to and from sub units. The hub units may distribute data packets received from a particular sub unit to another sub unit within the same coverage area via such frames, to a traditional wired network backbone, or to another hub unit.
A point to multipoint system, such as disclosed in the above referenced and commonly assigned patent application entitled “FREQUENCY REUSE FOR TDD,” contains a plurality of adjacently located hub units providing an aggregate coverage area. Additionally, these hubs may have their individual coverage areas divided into particular sectors—such as 30 or 90 degree sectors. Additionally, the hubs may utilize frequency division or other techniques to provide a plurality of communication channels.
Channel reuse techniques have developed to allow reuse of channels within a network without introducing unacceptable levels of interference. The purpose of these channel reuse techniques is maximize channel availability while avoiding co-channel interference between neighboring hubs. Clearly, these channel reuse techniques are valuable tools to increasing the bandwidth of point to multipoint systems. However, according to the present invention it has been realized that point to multipoint systems contain architectural characteristics that may be exploited to allow optimization of channel availability greater than that available with traditional channel reuse techniques while avoiding co-channel interference.
For example, data traffic over a point to multipoint system may be bursty, rather than at a fixed or continuous data rate. Specifically, an Internet browser application executed on a sub unit would typically require significant down link bandwidth while downloading HTML code from a website, but would require little or no bandwidth while a user reads the display associated with the HTML code. Additionally, the bandwidth requirements of many applications such as browsers may be asymmetric. Specifically, Internet browsers often download a large amount of data, but upload proportionally very little. Accordingly, point to multipoint systems may implement dynamic bandwidth allocation (DBA) techniques to maximize the data throughput associated with asymmetric, bursty traffic.
Accordingly, it is an object of the present invention to provide a system and method to maximize the bandwidth of point to multipoint systems in accordance with the unique characteristics of point to multipoint systems as between particular portions of the network.
It is an additional object of the present invention to provide a system and method for synchronized dynamic allocation of bandwidth.
It is an additional object of the present invention to provide a system and method for synchronization of receive and transmit modes of sectors or other portions of an associated group of hub units to maximize the bandwidth of point to multipoint systems.
It is an additional object of the present invention to provide a system and method for sector to sector telemetry in point to multipoint systems.
It is an additional object of the present invention to provide an efficient communication channel for use with the invention systems and methods that allows synchronization of neighboring hubs while permitting rapid dynamic allocation of bandwidth in individual hubs.
It is still an additional object of the present invention to provide a pattern of frequency re-use in a wireless communication system.
It is another object of the present invention to provide a repeatable pattern of frequency re-use in a wireless communication system comprised of sixteen cells in a four-by-four grid using two polarizations per communication frequency.
It is yet another object of the present invention to provide a repeatable pattern of frequency re-use in a wireless communication system comprised of sixteen cells grouped in four sub-clusters of four cells in which facing sectors in the pattern are synchronized.
It is a further object of the present invention to provide a method of reducing co-channel and/or adjacent channel interference by a pattern of frequency re-use.
These and other objects, features and technical advantages are achieved by a system and method which operate in a point to multipoint system comprising a plurality of hubs and a plurality of subs distributed within coverage areas associated with the hubs. The point to multipoint system preferably divides its communication bandwidth into channels utilizing spectrum division techniques, such as frequency division, time division, or orthogonal code division. Also, the hubs communicate to the subs within their coverage areas via sector antennae. By utilizing spectrum division and sector antennas, preferred embodiments of the point to multipoint system coordinate channel allocation via a channel reuse plan. Additionally, preferred embodiments divide individual channels into transmit and receive modes via a Time Duplex Division (TDD) scheme via the same channel. In this TDD scheme, a hub transmits information to subs in the transmit mode and receives information from subs in the receive mode. Moreover, the hubs of the point to multipoint system preferably may dynamically allocate bandwidth between the transmit and receive modes to achieve asymmetric communication modes. Also, the preferred embodiment subs utilizing the present invention comprise directional antenna.
Co-channel interference such as in adjacent sectors of neighboring hubs is a significant concern. Specifically, hub to hub exposure is problematic, since hub antennas are typically directed toward other hubs of the network in order to provide composite coverage of a service area. For example, preferred embodiment hubs may utilize sector antennas covering between 30 to 90 degrees in azimuth, which are oriented to face similar sector antennas at neighboring hubs. Sub unit exposure is not as a significant issue for the preferred embodiments point to multipoint systems, because sub units of these point to multipoint systems utilize highly directional antenna. Accordingly, the subs units may not be exposed to significant co-channel interference from other sub units or other hub units.
Channel reuse plans may be utilized to mitigate hub to hub co-channel interference. For example, by carefully assigning channels for use by the hubs of a network, reuse performance of approximately 1 may be achieved. Moreover, through advanced channel planning techniques, such as shown and described in the above referenced patent application, entitled “FREQUENCY REUSE FOR TDD”, and as described below, higher channel reuse performance may be achieved.
Nonetheless, a method or system optimization that would permit greater channel reuse would allow greater bandwidth for the system as a whole. The present invention achieves this goal in one embodiment by synchronizing transmit and receive modes of hubs. One embodiment of the present invention synchronizes dynamic bandwidth allocation of facing sectors of a cluster of geographically adjacent hubs, while allowing other sectors of these hubs to independently allocate bandwidth through frequency reuse and facing sector synchronization. The hubs are adjacent in the sense that the hubs are the nearest neighbor hubs in a particular direction. In this embodiment, guard time between transmit and receive modes is minimized by preferably selecting a guard time to accommodate the synchronization distance of just over two hub coverage radii. For example, where a maximum reuse is 6R, a reuse schedule of 9, with 30 degree sectors, 4.5 km cells, the guard time is approximately 100 μs or approximately 5% of the embodiment's channel capacity to accommodate propagation from a maximum distance in the reuse cluster. However, as the present invention synchronizes facing sectors of adjacent hubs, the synchronization distance is greatly reduced. Accordingly, in this embodiment, the guard time only occupies 0.5% of the channel capacity. Moreover, the computation requirements of the system are significantly reduced in this preferred embodiment, as a much smaller portion of the network is synchronized with respect to any particular synchronization determination. Also, the facing sector synchronization simplifies the implementation of synchronization telemetry.
In another embodiment of the present invention, a pattern of frequency reuse is described where a repeatable pattern of cells is employed to allow for re-use of a number of frequency assignments where there are two polarization modes available per frequency. Such a pattern of frequency re-use is especially useful when the number of frequency assignments, or communication channels, available for operation of a communication system is limited. In order to provide sufficient coverage for a particular operating area, a pattern of cells that re-use the available frequencies must be provided in order to avoid dead spots or to avoid interference between adjacent channels on the frequency spectrum used in the same area, known in the art as “adjacent channel interference” or interference between two cells using the same frequency with the same polarization in adjacent areas, known in the art as “co-channel interference”.
Idealizing the shape of the cells in the pattern as circular and further idealizing each cell as having a similar radius, the shape of a repeatable pattern of such cells can be viewed as an overlay on a flat surface. Obviously, such idealizations such as a flat surface and substantially identical cells spaced at uniform distances rarely occur in the real world. However, it is to be understood that the present inventive system and method is not limited to such idealizations but rather is applicable to real world situations where the overall frequency re-use pattern can be used while taking into account minor variations to allow for obstructions, terrain features, dissimilar cell sizes, irregular spacing of cells, etc. While the disclosure of the invention below will discuss an idealized repeatable pattern composed of idealized cells, etc., such idealizations should not be construed as limitations of the invention.
For cells of substantially the same size and circular in shape, one arrangement of those cells in a multi-cell pattern may be seen as a square grid where the edge of two cells that are adjacent in the same rank or the same file are tangent at one point. In such an arrangement, cells that are diagonally adjacent are not tangent. In another multi-cell arrangement, a cell in the pattern is tangent to each of six adjacent cells. Such a pattern would appear as a honeycomb shape if the cells are idealized to be hexagonal in shape.
The inventors have determined empirically that for cells with 90° sectors, a minimum of eight frequency assignments and two polarizations are required for efficient frequency re-use for broadband wireless access systems. This is a reasonable requirement of frequency/polarization assignments for 90° sectorized cells in a time division duplex (“TDD”) system considering the size of a typical license allocation of frequencies on a worldwide basis. For example, in Europe, the anticipated license allocation is 2×112 MHz or 224 MHz for the 28 GHz band and approximately 500 MHz for the 42 GHz band. Most of the North American broadband wireless access operators have allocations in excess of 200 MHz. An emerging popular channel size is 28 MHz in Europe and 25 MHz in North America. These channel sizes coupled with the anticipated license allocation of frequencies allows for eight or more available frequency channels.
While 90° sectors have some disadvantages over smaller sector sizes, such as 60°, 45°, and 30° sectors, 90° sector size is the baseline for planning for almost all broadband wireless access operators and standards groups. For example, RF performance is somewhat compromised for wide sectors relative to narrow sectors. Cell diameter is reduced thereby requiring a greater number of hubs/cells to cover a given area. Wider sectors also give rise to a greater possibility of co-channel and adjacent channel interference.
Despite the operational drawbacks of 90° sectors, there are significant economical advantages to 90° sector plans. One advantage is the lower cost of outdoor gear. With 90° sectors, fewer sectors and hence fewer radios, antennas, and associated equipment, both primary and redundant, are required when compared with smaller-sized sectors. Additionally, a significant cost to operators are roof rights. Landlords tend to charge for the right to place equipment of the roof of their building based on the number of antennas so 90° sectors translates into lower cost for roof rights. Also, wider sectors provide greater RF coverage which is an important benefit in the early deployment of a system.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention.